This invention relates to a method of making a paper support for photographic paper. More particularly, it relates to a method of making a paper support for photographic paper, which is excellent in uniform receptivity to emulsion coating and is free of ridge-like irregularities in the machine direction (hereinafter referred to briefly as surface irregularity) and free of separation of the paper support into two layers.
Photographic paper for color print or black and white print is generally processed in liquid media. As a consequence, a paper support of the photographic paper has been made of base paper having a sufficient wet strength. Baryta paper, which was widely used, is a paper support generally made by coating a wet-strength base paper with a coating composition comprising gelatin as binder and, dispersed therein, barium sulfate, an inorganic white pigment, and then drying. In recent years, with speed-up of the photographic processing, there has been chiefly used a paper support of resin-coated paper made by coating a base paper on both sides with a water-resistant resin.
The base paper for various paper supports is generally made by means of a Fourdrinier paper machine in which a web is formed by feeding a paper stock onto a travelling endless long wire cloth. The paper making speed has become higher to increase the productivity and various efforts have been made to keep the paper quality from decline caused by the high-speed paper making. Although not yet actually used in making photographic paper, a paper machine of the twin-wire type is used in making some of the newsprint and general printing paper. For example, according to U.K. Patent Application GB 2003952 A, a paper web is formed by dewatering a stock upwardly and downwardly simultaneously by using a top upwardly dewatering unit in conjunction with traditional downwards drainage. But, since a paper support for photographic paper is not aimed at by this U.K. patent application, a paper web with uniform distribution of fines and ash (clay) therein and a minimum two-sidedness is formed by adjusting the location of the top unit, for example, from 1 to 3 meters from a slice for feeding the paper stock. This U.K. patent application is quite silent on how to improve the smoothness of the surface of web on which a photographic emulsion layer is to be formed. When such a paper web is used for a paper support for photographic paper, there arise various problems, since there are many severe quality requirements in the production of photographic paper.
Improvements in productivity, for instance, the speed of coating the emulsion becomes higher and in making photographic paper for color print the conventional method of successively applying the required number of single coating layers is being superseded by the method of simultaneous coating of two or more emulsion layers or even simultaneous coating of total layer consisting of two or more emulsion layers. For such purposes, the support, not to speak of the emulsion, should meet severe quality requirements. In particular, the simultaneous total layer coating procedure suffers from the phenomenon called "shear in emulsion layers", that is, failure in uniformity of coating layers resulting from a delicate change in the thickness of a portion of emulsion layer, which leads to mottled surface of a color print, detracting much from the commercial value of the photographic paper.
Although the exact reasons for the phenomenon of shear in emulsion layers are yet to be elucidated, it is known that with the increase in speed of coating the emulsion, the shear in emulsion layers becomes more pronounced. The shear in emulsion layers is affected to some degree by the surface texture of the photographic paper, such as silk-finish, fine grained, matt, or glossy surface. In every case, however, the shear in emulsion layers tends to occur more easily with the increase in coating speed of the emulsion.
The factors of a support which affect the shear in emulsion layers include surface irregularities of the base paper used in making the support. The shear in emulsion layers decreases with the decrease in the degree of surface irregularities of the base paper. For instance, the phenomenon of shear in emulsion layers can be suppressed to a certain extent by calendering the base paper to increase the bulk density and, hence, to improve the surface irregularities. However, such a treatment is not sufficiently effective and, in addition, is not economical if it is necessary to increase the basis weight of base paper to retain required thickness of the support. If the web forming speed of the paper machine is increased, the resulting base paper exhibits increased surface irregularities, resulting in increased shear in emulsion layers. U.S. Pat. No. 4,582,785 to Woodward et al discloses polyolefin coated photographic base papers wherein a special stabilizer is used to prevent cracking in the image-containing layer, or even both the image-containing and the resin-containing layers. But Woodward et al have no inventive idea of having excellent surface characteristics (few surface irregularities) which allow high-speed coating of emulsions without exhibiting shear in emulsion layers.